Recently, as the CPU driving frequency has been becoming higher (e.g., 10 GHz or higher), attention has been directed toward optical interconnection techniques in which signals within and between system apparatus are transmitted by light. Semiconductor devices such as semiconductor photodetector devices and semiconductor light-emitting devices are used in the optical interconnection techniques.
When mountability to external substrates is concerned in a semiconductor photodetector device used in the optical interconnection techniques, it will be preferred if an electrode (signal electrode) for taking out signals from the photodetector device is arranged on a surface opposite from a light-incident surface. Examples of such semiconductor photodetector devices are disclosed in Japanese Patent Application Laid-Open Nos. HEI 3-104287, HEI 6-296035, and 2002-353564. These publications disclose semiconductor photodetector devices of back-illuminated type in which a plurality of compound semiconductor layers are formed on one main face side of a semiconductor substrate, while light is incident from the other main face side.
For the following purposes, these back-illuminated semiconductor photodetector devices partly thin the portion of the substrate located under the photodetecting part, while surrounding this portion with a part maintaining the thickness of the substrate. The first purpose is to prevent signals from deteriorating or disappearing because of light absorption by the semiconductor substrate. The second purpose is to prevent the semiconductor photodetector devices from being damaged or broken when mounting the semiconductor photodetector devices onto external substrates by wire bonding or bump bonding.
However, there is a limit to reducing the size of the above-mentioned back-illuminated semiconductor photodetector devices, since there is a portion maintaining the substrate thickness in order to keep mechanical strength. When forming an array of semiconductor photodetector devices by providing a plurality of photodetecting parts in particular, the pitch between the photodetecting parts is hard to narrow, whereby the semiconductor photodetector device array must increase its size.